This application is based on and claims priority under 35 U.S.C. xc2xa7119 with respect to Japanese Patent Application 2001-281203, filed on Sep. 17, 2002, the entire content of which is incorporated herein by reference.
This invention relates to a hydraulic brake apparatus for a vehicle. Particularly, this invention pertains to a hydraulic brake apparatus suitable for an electric vehicle to perform regenerative braking.
A known hydraulic brake apparatus for a vehicle is disclosed in Japanese laid-open publication No 3(1991)-45456 published on Feb. 27, 1991. This hydraulic brake apparatus includes a hydraulic pressure generator for generating a power pressure irrespective of operation of a brake pedal and outputs the power pressure. A regulation valve regulates the power pressure outputted from the hydraulic pressure generator to a predetermined pressure corresponding to an operation force of the brake pedal and outputs the predetermined pressure to an output chamber. A master cylinder has a master cylinder piston and a master cylinder pressure chamber located forward the master cylinder piston to generate a master cylinder pressure by forward movement of the master cylinder piston. A wheel brake cylinder is operated by the master cylinder pressure to apply braking force to a wheel of the vehicle. An auxiliary piston is located backward the master cylinder piston to define an auxiliary pressure chamber between the master cylinder piston and the auxiliary piston. The auxiliary pressure chamber communicates with the output chamber of the regulation valve to move the master cylinder forward. When both of the hydraulic pressure generator and the regulation valve are normal condition in which the predetermined pressure can be outputted to the output chamber of the regulation valve, the auxiliary piston is biased backward by the auxiliary pressure in the auxiliary pressure chamber and kept in the initial position. In this hydraulic brake apparatus, in the normal condition, the operation (depression) force of the brake pedal which is necessary for applying a certain braking force to the wheel is relative small.
On the other hand, when at least one of the hydraulic pressure generator and the regulation valve is abnormal condition in which the predetermined pressure cannot be outputted to the output chamber of the regulation valve, the auxiliary piston is moved forward from the initial position by the operation force of the brake pedal to move the master cylinder piston forward. In detail, in this abnormal condition, the auxiliary pressure is not generated in the auxiliary pressure chamber and thus the auxiliary piston is not biased backward by the auxiliary pressure. If the brake pedal is operated, the auxiliary piston is moved forward by the braking operation force and contact to the master cylinder piston to move master cylinder piston forward. As a result, the braking pressure can be supplied to the wheel brake cylinder.
An electric vehicle driven by an electric motor is known. In this electric vehicle, both regenerative braking force generated by the motor and hydraulic braking force generated by the hydraulic pressure in the wheel brake cylinder are cooperatively applied to the wheel to increase the life of a battery which is an electric source of the motor (regenerative cooperation brake). In the electric vehicle, when the brake pedal is operated, the regenerative braking force must be taken priority over the hydraulic braking force for recovering the wasteful energy. Thus, it is necessary to decrease the hydraulic pressure supplied to the wheel cylinder by a hydraulic pressure corresponding to regenerative braking force in order to generate an appropriate braking force corresponding to the operation force of the brake pedal or braking force required by a driver.
However, in the above known hydraulic brake apparatus, because the auxiliary pressure chamber communicates with the output chamber of the regulation valve, the auxiliary pressure in the auxiliary pressure chamber is equal to the pressure in the output chamber corresponding to the braking operation force. For that reason, the master cylinder pressure corresponding to the braking operation force is supplied to the wheel cylinder. Thus, if the known apparatus is applied to the electric vehicle performing the above regenerative cooperation brake, a braking force greater than that required by the driver is applied to the wheel by the regenerative braking force. Thus, it is necessary to add a pressure modulation device which decreases the predetermined pressure outputted from the output chamber of the regulation valve by the hydraulic pressure corresponding to the regenerative braking force for supplying the decreased pressure to the auxiliary pressure chamber in order to solve the drawback of the known hydraulic brake apparatus.
However, if a larger regenerate braking force is required in dependence on a vehicle condition such as a vehicle speed, the pressure supplied to the auxiliary pressure chamber by the pressure modulation device becomes lower and thus biasing force for biasing the auxiliary piston backward by the pressure in the auxiliary pressure chamber becomes smaller. In this condition, even if both of the hydraulic pressure generator and the regulation valve are the normal condition in which the predetermined pressure can be outputted to the output chamber of the regulation valve, the auxiliary piston may be moved forward unnecessarily by the braking operation force and thus the master cylinder piston may be moved forward unnecessarily. As a result, a superfluous master cylinder pressure and superfluous hydraulic braking force may be generated.
A need exists for a vehicle hydraulic brake apparatus which is not susceptible to the drawbacks mentioned above.
A hydraulic brake apparatus for a vehicle includes a hydraulic pressure generator which generates a power pressure irrespective of operation of a brake pedal and outputs the power pressure and a regulation valve which regulates the power pressure outputted from the hydraulic pressure generator to a predetermined pressure corresponding to an operation force of the brake pedal and outputs the predetermined pressure to an output chamber. A master cylinder has a master cylinder piston and a master cylinder pressure chamber located forward the master cylinder piston to generate a master cylinder pressure by forward movement of the master cylinder piston. A wheel brake cylinder is operated by the master cylinder pressure to apply braking force to a wheel of the vehicle. An auxiliary piston is located backward the master cylinder piston to define an auxiliary pressure chamber between the master cylinder piston and the auxiliary piston and to move the master cylinder forward by an auxiliary pressure in the auxiliary pressure chamber. A pressure modulating device modulates the auxiliary pressure in the auxiliary pressure chamber to a pressure less than the predetermined pressure in the output chamber of the regulation valve. A pressure chamber communicates with the output chamber of the regulation valve to bias the auxiliary piston backward by a pressure in the pressure chamber. The auxiliary piston is biased backward by both the auxiliary pressure in the auxiliary pressure chamber and the pressure in the pressure chamber when both of the hydraulic pressure generator and the regulation valve are a normal condition in which the predetermined pressure can be outputted to the output chamber of the regulation valve, and the auxiliary piston is moved forward by the operation force of the brake pedal to move the master cylinder piston forward when at least one of the hydraulic pressure generator and the regulation valve is an abnormal condition in which the predetermined pressure cannot be outputted to the output chamber of the regulation valve.